Carbonate facies evolution from the late albian to ... - Springer Link

FACIES

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PI. 38-41

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ERLANGEN 1998

Carbonate Facies Evolution from the Late Albian to Middle Cenomanian in Southern Istria (Croatia): Influence of Synsedimentary Tectonics and Extensive Organic Carbonate Production Josip Ti~ljar, Igor Vlahovi6.., Ivo Veli(., Dubravko Matinee, Zagreb and Julie Robson, Southminster KEYWORDS: PERITIDAL CARBONATES - TEMPESTITES - SUBAQUEOUS DUNES - RUDIST BIOSTROMES SYNSEDIMENTARY TECTONICS - ADRIATIC CARBONATE PLATFORM - SOUTHERN ISTRIA (CROATIA) - UPPER ALBIAN TO MIDDLE CENOMANIAN SUMMARY During the Late Albian, Early and Middle Cenomanian in the NW part of the Adriatic Carbonate Platform (presentday Istria) specific depositional systems characterised by frequent lateral and vertical facies variations were established within a formerly homogeneous area, ranging from peritidal and barrier bars to the offshore-transition zone. In southern Istria this period is represented by the following succession: thin-bedded peritidal peloidal and stromatolitic limestones (Upper Albian); well-bedded foreshore to shoreface packstones/grainstones with synsedimentary sliding and slumping (Vraconian - lowermost Cenomanian); shoreface to off-shore storm-generated limestones (Lower Cenomanian); massive off-shore to shore face carbonate sand bodies (Lower Cenomanian); prograding rudist bioclastic subaqueous dunes (Lower to Middle Cenomanian); rudist biostromes (Lower to Middle Cenomanian), and high-energy rudist and ostreid coquina beds within skeletal wackestones/packstones (Middle Cenomanian). Rapid changes ofdepositional systems near the Albian/ Cenomanian transition in Istria are mainly the result of synsedimentary tectonics and the establishment of extensive rudist colonies producing enormous quantities of bioclastic material rather than the influence of eustatic changes. Tectonism is evidenced by the occurrence of sliding scars, slumps, small-scale synsedimentary faults and conspicuous bathymetric changes in formerly corresponding environments. Consequently, during the Early Cenomanian in the region of southern Istria, a deepening of the sedimentary environments occurred towards the SE, resulting in the establishment of a carbonate ramp system. Deeper parts of the ramp were below fair-weather wave base (FWWB), while the shallower parts were characterised by high-energy environments with extensive rudist colo-

nies, and high organic production leading to the progradation of bioclastic subaqueous dunes. This resulted in numerous shallowing- and coarsening-upwards clinostratified sequences completely infilling formerly deeper environments, and the final re-establishment of the shallow-water environments over the entire area during the Middle Cenomanian.

1 INTRODUCTION The Istrian peninsula (Fig. I ) represents the NW part of the Adriatic Carbonate Platform. This platform is composed of a succession of carbonate deposits more than 2,000 m thick (mostly limestones, rarely dolomites and carbonate breccias) of Middle Jurassic to Eocene age, and is overlain by Eocene flysch deposits (PoL~AK, 1965a, 1965b; TI~LJAR, 1978; VELIr & TI~LJAR, 1988: TI~L.1AR VELIC, 1991 ). Jurassic and Lower Cretaceous deposits of Istria, ranging from Bathonian to Upper Albian age, are predominantly characterised by shallow-marine deposition, only sporadically interrupted by periods of emersion (TI~LJAa, 1978; TI~gLJARet al., 1983; VELII2& TI~;LJAR,1987; VEL[C et al., 1995a). This succession is divided into several laterally continuous units which exhibit the gradual changes typical of the facies diversity on carbonate platforms. By the end of the Albian and during the Cenomanian discrete depositional systems were established in different parts of the platform, which were characterised by lateral changes from peritidal and barrier bars to gently inclined carbonate ramp deposits across present-day Istria (VLAHOVIr et al., 1994; TI~LJARet al., 1995). These changes also occurred in Upper Albian and Lower- Middle Cenomanian deposits at other localities on the Adriatic Carbonate Platform.

Addresses: Prof. Dr. J. Tigljar, Faculty of Mining, Geology & Petroleum Engineering, University of Zagreb, Pierottijeva 6, P.O. Box 679, HR-10000 Zagreb, Croatia. M.Sc. I. Vlahovi6, Dr. I. Veil6, Dr. D. Mati~ec, Institute of Geology, Sachsova 2, P.O. Box 268, HR-10000 Zagreb, Croatia [E-mail: geologia-croatica @zg.tel.hr]. Dr. J. Robson, Demandkey Ltd., 2 Newmoor Cottages, Northwycke, Southminster, Essex, CMO 7DR, United Kingdom.

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Fig. 1. Schematic distribution of megasequences in lstria and location of the Western Istrian anticline (after VELI~ et al., 1995a).

2 GEOLOGICAL SETTING The most important geological structure of the Istrian peninsula is the Western Istrian anticline (Pot~AK & ~IKI~, 1973; MARIN~I~ & MATIdEC, 1991), as shown on Fig. 1. Unlike other areas of the Adriatic Carbonate Platform, where the effects of Cretaceous tectonism have mostly been destroyed by later tectonic activity, these are still visible in Istria (MATIdZCet al., 1996). The carbonate deposits of Istria are divided into four megasequences (Fig. 1; VELI~et al., 1995a), each of which was terminated by an important, long-lasting emersion, i.e. by a type I sequence boundary (Fig. 2). Albian Middle Cenomanian deposits forming the subject of this study comprise a part of the third megasequence (Fig. 2). 2.1 The Bathonian - Early Kimmeridgian regressive megasequence

This megasequence is approximately 200 m thick (Fig. 2), and is characterised by shallowing- and coarseningupward trends. It is terminated by the Kimmeridgian Early Tithonian emersion with bauxite deposits. This

megasequence is divided into four units representing lowenergy subtidal environments and prograding high-energy tidal bars, as well as shoreline and terrestrial environments at the top of the succession (VEu~ & T~t.JAR, 1988; TI~LJAR et al., 1994). 2.2 The Late Tithonian - Late Aptian transgressiveregressive megasequence

This megasequence is very complex (Fig. 2), because of its facies heterogeneity and considerable thickness (465-545 m). Mostly composed ofperitidal shallowing-upward metre scale sequences, the beginning of the sequence is marked by the "oscillating transgression" (TI~LJARet al., 1983) at the end of the Tithonian, and the end by the regional Late Aptian emersion (VEm~ et al., 1989). 2.3 The Late Albian - Early Campanian transgressive-regressive megasequence

This megasequence is more than 1,000 m thick and composed of a variable facies succession (Fig. 2). After extensive emersion during the Late Aptian and Early

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Albian, i.e. by the beginning of the Late Albian, a transgression occurred which was at first gradual, (marked by 3-5 short emersions containing smectite clay of probable volcanic origin - DUaN et al.. 1997), culminating with the later re-establishment of the shallow-water platform carbonate system in the Istrian part of the Adriatic Carbonate Platform. This megasequence comprises several lithostratigraphic units (VEkIC et al., 1995a), including the peritidal and foreshore sedimentary system during the Albian, differentiation of sedimentary systems during the Upper Albian to Middle Cenomanian, the drowned platform system during the youngest Cenomanian and Turonian (JENKYNS, 1991; Gu.~Ir & JELASKA, 1993; VLAHOVleet al., 1994) and re-establishment of the shallow-water sedimentary system during Late Turonian, Coniacian and SantonianCampanian. The Upper Albian to Middle Cenomanian succession in southern Istria (Veli Brijun Island, Banjole, Fragkulin Island, Vinkuran and Pomer; Fig. 1, 3) is the subject ofthe present study. 2.4 Palaeocene - Eocene Carbonate and Clastic Sequences

The Palaeocene - Eocene stratigraphic succession in lstria comprises a relatively thick succession of carbonate and clastic rocks (Fig. 2). The duration of the emersion, locally with bauxite deposits, between the Upper Cretaceous and Palaeogene differed from area to area (MATICEC et al., 1996). Therefore, members of the Palaeogene deposits were transgressively deposited on different members of the Cretaceous basement. The succession of Palaeogene deposits is very variable in both the lateral and vertical sense, and can, in general, be divided into several units (Fig. 2).

3 F A C I E S AND D E P O S I T I O N A L E N V I R O N M E N T S OF THE UPPER ALBIAN TO MIDDLE CENOMANIAN CARBONATES IN S O U T H E R N ISTR1A The Upper Albian, Lower and Middle Cenomanian carbonate deposits of southern Istria (in the area of the Veli Brijun Island, Ban.jole, Fragkulin Island, Vinkuran and Pomer) can be divided into seven facies units (Fig. 3). 3.1 Facies Unit 1: Thin-bedded peloidal and stromatolitic limestones (Upper AIbian)

Fig. 2. Schematic geological column oflstria showing fouremersionbound megasequences,

Upper Albian carbonate deposits of southern lstria are mostly represented by thin-bedded (5-20 cm) peritidal limestones. Late-diagenetic dolomitisation is sporadic. and at some localities the deposits are almost completely dolomitized into hypidiomorphic micro- to macrocrystalline dolomites with mosaic structure (TIgLJAR, 1977). This facies unit is characterised by the predominance of fine-grained, well-sorted peloidal wackestones and

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Fig. 3. Geological column of the Upper Albian to Middle Cenomanian carbonate succession in southern Istria showing the seven facies units described in the text. packstones alternating with mudstones, and sporadic LLHstromatolites and intraformational breccias (Fig. 3). Stromatolites are composed of uneven, wavy, frequently disturbed cyanobacterial laminae, and thinner ostracod wackestones, peloidal packstone/grainstones and sporadic mudstone laminae. Some cyanobacterial laminae contain desiccation cracks (PI. 38/1), while others are disrupted and reworked by wave and tide currents into stromatolite intertidal breccia. Stromatolite members are occasionally covered by thin layers of fine-grained current-rippled peloidal packstone/grainstone (P1. 38/2). Deposits of Facies Unit 1 contain small gastropods, ostracods and an assemblage of Late Albian benthic fora-

minifera (Neoiraquia insolita (DECROUEZ• MOULLADE), Valdanchella dercourti DECROUEZ & MOULLADE,Paracoskinolina fleury DECROUEZ & MOULLADE,Cuneolina pavonia D' ORmGNY,Cuneolina parva HENSON,Vercorsella laurentii (SARTONI & CRESCENTI),Sabaudia auruncensis (CHIoCCI-IINI & DI NAPOLI), Pseudonummoloculina heimi (BONET)), and numerous shells of pachydont molluscs in sporadic coquina layers (VELI(~& TIlLJAR, 1987; VELI,~et al., 1995b). Dinosaur footprints have been found in two levels of this unit on the island of Veli Brijun (Fig. 1) (BACHOFEN-ECHT, 1925, 1926; POL~AK, 1965b; VELtC & TI~LJAR, 1987; DALLAVECCHIA& TARLAO, 1995). Limestones of this facies unit are interpreted as peritidal

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P 1 a t e 38

Carbonate facies evolution from the Late Albian to Middle Cenomanian in Sottthern Istria (Croatia)

Fig. 1. Desiccation cracks on the weathered upper surface of a peloidal wackestone bed; top of the shallowingupward cycle; Upper Albian, Facies Unit 1, Veli Brijun island. Lens cap in the lower left is 52 mm in diameter. Fig. 2. Peloidal packstone with small-scale current ripples; Upper Albian, Facies Unit 1, Veli Brijun island. Width of the outcrop is approximately 3.5 m. Fig. 3. Thin-bedded laminated peloidal and stromatolitic wackestone/packstones with an infilled sliding scar: A) a photograph (infilled scar is dotted, lens cap is 52 mm in diameter); B) schematic interpretation of sliding and slumping. Vraconian - Lowermost Cenomanian, Facies Unit 2, Banjole.

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deposits with common shallowing-upward cycles of decimetre scale, being composed of two members: mudstone and peloidal wackestone/packstone deposited in a low-energy subtidal environment, and peloidal wackestones with stromatolitic laminae or LLH stromatolites deposited in intertidal (to supratidai?) environments. Some cycles end with desiccation cracks (PI. 38/1), an erosional surface or a intraformational peritidal breccia of variable lateral thickness (tidal channel and/or storm tide deposits - VELlt~ t~r TIgL1AR,1987). Shallowing-upward cycles are characterised by the gradual increase of the subtidal and decrease of the intertidal portion of successive cycles.

3.2 Facies Unit 2: Well-bedded paekstones/grainstones with sliding and slumping features (Vraconian - Lowermost Cenomanian) The transition from thin-bedded peritidal peloidal and stromatolitic limestones of Facies Unit 1 to well-bedded (~ 20 cm) laminated wackestone/packstones with thin layers of peloidal grainstones of Facies Unit 2 is gradual. In the transitional zone between these two facies units infrequent thin stromatolite layers occur. Further up the succession, stromatolites are completely missing, while the portion of thicker (30-40 cm) medium to coarse-grained intraclasticpeloidal grainstones containing foraminifera and infrequent large (0.5 to 4 mm) micritic intraclasts gradually increases. The microfossil assemblage of the Facies Unit 2 is characterised by ostracods and Thaumatoporella, while small benthic foraminifera (mostly miliolids) are very infrequent. Facies Unit 2 (Fig. 3) is characterised by specific depositional structures indicating changes of depositional conditions: slides, slumps, and small-scale synsedimentary faults (PI. 38/3, P1.39/1, 2). Such deformational features are present exclusively in certain horizons within an otherwise concordant succession. Sliding scars are up to 1.8 m long, and up to 30 cm deep. The depression formed by displacement caused by sliding (lst stage, PI. 38/3B) is infilled by several thin (1-6 cm), lenticular beds of laminated coarse-grained peloidal and intraclastic packstones of the hanging-wall bed (2nd stage, P1.38/3B). Lenticular beds are thickest in the central parts of the depression, and thin towards the margins, sometimes completely pinchP 1 a t e 39 Fig. 1. Fig. 2.

Fig. 3. Fig. 4.

ing-out. Laterally, down the slope, deformed beds are characterised by small-scale faults and slumps, as well as structures formed by loading into the incompletely consolidated footwall sediments, similar to ball-and-pillow structures. Sliding and slumping of unconsolidated sand beds was probably caused by seismic shocks, recorded as evidence of synsedimentary tectonic activity (PI. 38/3, PI. 39/1, 2). Undisturbed thicker packages (1 to 1.6 m thick) indicate phases of relative tectonic calm. Tectonic activity was a consequence of the formation of a gentle anticline in the hinterland (1 st stage, Fig. 4), causing a gradual inclination of the platform towards the ESE (- 115" direction) in the study area. Orientations of the small-scale slump movements (Fig. 5) correspond to the general bedding (120/6). Small-scale faults are further evidence of synsedimentary tectonics and show displacements of up to 10 cm. These normal faults are the result of movement between beds, with the common dragging of incompletely consolidated sediment, as evidenced by small folds in the hanging wall which have orientations corresponding to the slumps. The measured structural elements of these faults are shown in Fig. 5. Synsedimentary tectonics represents part of the regional compressional deformation, which resulted in the uplift of the NW hinterland of the study area. It is important to stress that the orientation of the b-axes of all the aforementioned structures corresponds to the b-axis of the macrostructure - the West Istrian anticline, which was finally shaped during the Laramian tectonic phase at the end of the Cretaceous. However, tectonic activity with the same orientation as the Laramian tectonic phase on the Istrian part of the Adriatic Carbonate Platform commenced much earlier (MATlg?ECet al., 1996). Facies Unit 2 represents a gradual transition from peritidal environments of the Facies Unit 1 to shoreface to offshore environments of the Facies Unit 3 (Fig. 3).

3.3 Facies Unit 3: Storm-generated limestones (Lower Cenomanian) Transition to Facies Unit 3 is characterised by a change from the thin-bedded limestones of Facies Unit 2 into swaley and hummocky cross-stratified deposits. Well sorted, very fine grained (0.03-0.1 mm), intraclastic/ peloid packstone and grainstone with sporadic small benthic

Carbonate facies evolution from the Late Albian to Middle Cenomanian in Southern Istria (Croatia) Small-scale synsedimentary overthrust with slump structure in the front. Vraconian - Lowermost Cenomanian, Facies Unit 2, Banjole. Scale bar = 5 cm. Small-scale synsedimentary normal fault - note gradual decrease of displacement along the fault plane. Vraconian - Lowermost Cenomanian, Facies Unit 2, Banjole. To the right of the compass is a 5 cm long scale bar. Hummocky cross-stratified peloid packstones (tempestites); Lower Cenomanian, Facies Unit 3, Banjole. 7 cm long hand lens for scale. Abraded orbitolinid tests in well-sorted fine-grained pelmicrite matrix. Lower Cenomanian, Facies Unit 3, Banjole. Width of the photo is 13 mm.

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Fig. 4. Schematic interpretation of the facies succession from the Late Albian to Middle Cenomanian in southern Istria.

P 1 a t e 40

Carbonate facies evolution from the Late Albian to Middle Cenomanian in Southern Istria (Croatia)

A) Schematic drawing of the outcrop on the Fragkulin Island showing the succession of facies units. Numbers at the bottom indicate the measured inclination of beds; B) a photograph of the part of the outcrop showing foresets of subaqueous dune (Facies Unit 5) downlapping on gently inclined footwall deposits (Facies Units 2 to 4); width of the outcrop is approximately 115 m.

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Fig. 5. Stereographic projection of the measured structural elements in Facies Unit 2 deposits: traces of normal fault plains (lines), orientation of slump movements [,arrows) and pole of general bedding (black circle). foraminifera and bioclasts of thin-shelled molluscs are characterised by hummocky cross-stratification which is clearly delineated by alternation of thin (< 1 mm thick) laminae. While the P, F and X members of the idealised tempestite sequence (Do'r'r & BOURCEOtS, 1982) are not completely developed or are very thin, the H member, characterised by hummocky cross-lamination is the thickest (20-60 cm) and for carbonate deposits well developed (P1.39/3). This is the consequence of common amalgamation caused by recurring storms in the shallowest part of the offshoretransition zone and deeper part of the shoreface, i.e. in the zone near the fair-weather wave-base (lst stage, Fig. 4). The upper bedding surface of the hummocky crossstratified limestone is erosional, characterised by a basal lag composed of coarse rudist bioclasts and complete rudist shells (10-30 cm in size). This is overlain by a 180 em thick sequence of nodular wackestone/packstones. In the lower part of this package the nodules are 5- I 0 cm thick, and 10-15 em wide. In the upper part they are 3-5 cm thick with the same width, being more discoidal. The upper third of this package is intensely bioturbated. Nodules are composed of well-sorted, fine-grained pelmicritic matrix and variable, but relatively high proportions of abraded orbitolinid tests (PI. 39/4) and rudist bioclasts. Orbitolinids (predominantly Orbitolina (Conicorbitolina) conica (D'~kRCHIAC)), indicate a definite Cenomanian age for the first time in this succession. ForaminiferaI tests are easily winnowed by storm waves (LI et al., 1997), and were concentrated below the fair-weather wave-base as the storm waned. The second nodular package is approximately 170 cm thick, and in the lower part consists of a 30 cm thick basal lag composed of grainstone/rudstone with gastropods and rudist bioclasts (Fig. 3). The upper part is also characterised by horizontal bioturbation.

Nodules of both nodular packages are probably relics of deformed, amalgamated tempestite sequences characterised by hummocky cross-bedding. Amalgamation is typical for lower shoreface deposits, and the beginning of the regressive trend in the depositional system (CHEEL& LACKIE,1993). The accumulation of large rudist shells and bioclasts in the form of the basal lag, as well as their predominance in the fine-grained biodetritus in the following parts of succession, indicate the contemporary existence of rudist colonies in neighbouring shallow-water areas. The material was transported by storm currents, and the structure and composition of tempestite sequences indicates that the large quantity of material was rapidly deposited near the fair-weather wave base, probably as a consequence of the introduction of sediments by earthquakes or other events that caused liquefaction during the storm events (cf. MYRow & SOUTHARD, 1996). Such rapid deposition of a relatively thick succession resulted in a lack of intense reworking by shallow-water mechanisms even above the FWWB. The upper parts were intensely bioturbated during the prolonged periods of fair weather characterised by low deposition rates. Deposits of Facies Unit 3 are interpreted as being of shoreface to offshore origin (Fig. 3). Rapid deposition of a large quantity of material which originated from the very productive shallow areas overrode local subsidence rates, resulting in the beginning of infilling of the formerly deeper area formed by the synsedimentary tectonics. Therefore, this unit can be attributed to the beginning of the regressive trend in the Albian to Cenomanian succession in southern lstria. 3.4 Facies Unit 4: Massive carbonate sand bodies (Lower Cenomanian) Facies Unit 4 consists of a 6-15 m thick massive carbonate sand bodies (Fig. 3, 2nd stage on Fig. 4) generally characterised by a coarsening-upward sequence. It is mostly represented by well-sorted fine-grained packstones and grainstones composed of rudist bioclasts, peloids, echinoderm fragments and coarse orbitolinid tests. Grains are mostly cemented by microcrystalline calcite cements, although there is a variable proportion of micrite matrix. The uppermost part is intensely bioturbated. The material originated from two sources: peloids and benthic foraminifera from the shallow subtidal area, and rudist debris (increasing towards the upper part) from reworked rudist colonies. Carbonate sand bodies are massive, lacking any internal structure, probably as a consequence of their being well-sorted. Facies Unit 4 represents a gradual transition from shoreface to offshore environments of the Facies Unit 3 to the clinostratified bodies of the Facies Unit 5 (Fig. 3). 3.5 Facies Unit 5: Prograding rudist bioclastic clinostratified bodies (Lower to Middle Cenomanian) This facies unit (Fig. 3) is characterised by rudist clinostratified bodies formed bY deposition of debris and

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Fig. 1.Radiolitid colony with specimens in growlh position. Upper bedding surface of rudist biostrome; Lower to Middle Cenomanian, Facies Unit 6, Fragkulin Island in the Banjole Cove; lens cap: ~ 52 ram. Fig. 2. Tempestite ostreid coquina between two beds of rudist coquinas. Middle Cenomanian, Facies Uni! 7, Pomer Cove (SE of Pula); scale is 22 cm long infrequent complete shells (2nd stage, Fig. 4). Inclination of the clinostratified sets varies from 10-15 ~ with a gradual decrease towards the SE, i.e. towards the younger bodies (PI. 40). Facies Unit 5 is composed of numerous clinostratified bodies, the thickness of which decreases from the shallowwater area (average thickness 2-6 m) towards the sea where they pinch-out (0-0.2 m). Each body is characterised by a fining-upward sequence, usually composed of coarsegrained debris or complete rudist shells in the lower part (above a more or less obvious erosional surface), while the fine-grained matrix increases towards the upper part. How-

ever, the complete succession is characterised by a coarsening-upward trend, since in successive clinostratified bodies the proportion of coarse-grained material gradually increases. The bodies are classified as rudist-ostreid floatstones consisting of poorly sorted coarse fragments and shells in a fine-grained matrix composed of well-sorted mollusc debris and peloids. Cementation is mostly incomplete: part of the pore space is filled by microcrystalline calcite cement, part by micrite matrix, the remainder being open. Bioclasts are unabraded, indicating short transport and a local sediment source.

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Fig. 6. Correlation scheme of the Upper Albian and Cenomanian carbonate deposits in northern and southern Istria (simplified after VLAHOVt~et al.. 1994). Clinostratified bodies of Facies Unit 5 are interpreted as foresets of subaqueous dunes, being a consequence of storm and wave reworking of highly productive rudist colonies, and the redistribution of the material into neighbouring areas (2nd and 3rd stages, Fig.4). Such bioclastic bars are typical progradational features, resulting from downlapping into formerly deeper areas. Since the contemporaneous shallow-water carbonate system (rudist colonies) was very productive, deposition rates were very high, and the succession of numerous clinostratified bodies resulted in the shallowing-upward trend, followed by the lateral shift of rudist colonies (Facies Unit 6). Large subtidal carbonate dunes are poorly understood, making their deposits difficult to identify and interpret (ANASTASet al., 1997). However, the position of these bodies within the succession in southern Istria (between

shoreface - off-shore tempestites and prograding rudist biostromes and peritidal deposits; Fig. 3) indicates a shallow carbonate ramp setting. 3.6 Facies Unit 6: Rudist biostromes (Lower to Middle Cenomanian)

Facies Unit 6 (Fig. 3) comprises extensive rudistid biostromes with rudists in their primary growth positions (PI. 41/1). In addition to frequent radiolitid rudists (usually 10-15 cm long shells), scarce nerineids and chondrodonts have been observed. The remaining pore space is filled with a fine-grained bioclastic packstone/grainstone matrix. Deposits of this unit are well exposed on the cliff of the Fra~kulin Island (PI. 41/1). However, these outcrops are

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intensely karstified and weathered, and are composed of numerous small elevators (which are better preserved) and some recumbent forms (which are mostly leached, since they were probably originally largely aragonitic - Ross & SKELTON, 1993). The best outcrops are in the Vinku,'an quarry, known from Roman times, where an almost 50 m high vertical cut is exposed, comprising Facies Units 4, 5 and 6. Among the rich macrofossil assemblage POLgAK (1965b) determined numerous rudists of the genera

Radiolitides, Sauvagesia, Ichthyosarcolites, Gyropleura, Monopleura and Praeradiolites, as well as the chondrodonts Chondrodonta joannae (CHOVFAT), Ch. joannae angusta (ScHuBER'I'), Ch. munsoni HtLL and gastropods Nerinea olisiponensis SHARPEand N. nobilis MU~S~R. These findings are insufficient to determine the age of the deposits of Facies Units 4, 5 and 6, since there are no accompanying microfossils, which are much more important for Cenomanian biostratigraphy. However, in the layers just above the top of the Facies Unit 6 a benthic foran'finifera Cho,salidina gradata D'OR~tGNY has been found, which is typical l-or the Middle and Upper Cenomanian of the wider Mediterranean realm, as well as of northern Istria (VEt.I(2& VLAHOV~r 1994). Since Facies Unit 3 is of Lower Cenomanian age, it is probable that the Facies Units 4, 5 and 6 are of Lower Cenomanian age as well, with a possible transition to the Middle Cenomanian. Deposits of Facies Unit 6 are interpreted as in situ rudist colonies from shallow subtidal environments prograding over foresets of subaqueous dunes.

3.7 Facies Unit 7: Rudist and ostreid coquina beds within skeletal wackestones/packstones (Middle Cenomanian) Facies Unit 7 overlies rudist biostromes of Facies Unit 6, but also represents its lateral equivalent (Fig. 3). It is mostly composed of the alternation of thick-bedded rudist coquinas with 0.3-1.2 m thick layers of ostreid coquinas (Pt. 41/2) and skeletal wackestone to packstones. The rudist coquinas are composed of weakly sorted rudist shells and coarse bioclasts ofrudists and rarely chondrodonts, as well as gastropod and other mollusc bioclasts in a finegrained bioclastic matrix and/or coarse-crystalline calcite cement. Ostreid coquinas are composed of complete shells or coarse fragments of ostreids (10-30 cm size), which are mostly subparallel and convexly orientated (P1. 41/2). Skeletal wackestone to packstones, rarely mudstones, contain a microfossil assemblage typical of the Middle to Late Cenomanian (VELI(2& VLAHOVI(2,1994) including Broeckbla (Pastrikella) balcanica Ch'ERCHtet al., Cho'salidma gradata D'ORmGNY,Pseudolituonella reicheliMAalE, and frequently Nummoloculina regularis PHILIPPSON. Rudist and ostreid coquinas were formed by wave and tide abrasion and reworking of rudist and ostreid colonies during high-energy periods, and were redeposited in shallow-water environments dominated by bioclastic mudsupported limestones.

4 D I S C U S S I O N AND C O N C L U S I O N S While the Lower Cretaceous deposits of Istria are predominantly characterised by uniform peritidal - lowenergy lagoonal depositional systems, the transition from the Early to kate Cretaceous is characterised by distinct facies variability. Therefore, it is necessary to concisely describe the situation in other parts of the Adriatic Carbonate Platform of present-day Istria during this period. During the kate Albian the whole area was still characterised by more or less stable peritidal - foreshore environments (Fig. 6). In southern Istria the)' are represented by Facies Unit 1 (section 3. I ). Only the central parts of Istria were subaerially exposed until the Palaeogene (MATI~'ECet al., 1996). The transition fi'om the Early to Late Cretaceous (Vraconian - beginning of the Early Cenomanian) is marked by the establishment of different sedimentary environments in northern and southern Istria (Fig. 6; VLAnOWC et al., 1994). In the western part of northern Istria (column 1, Fig. 6) peritida[ deposition continued in the Early and Middle Cenomanian. In the central part of northern Istria (column 2. Fig. 6) a large prograding carbonate sand body was deposited in the upper shoreface and foreshore environments, while in the eastern part of northern Istria (column 3, Fig. 6), contemporaneous sediments were deposited on a gently inclined inner carbonate ramp. By the end of the Cenomanian this highly differentiated depositional area was in filled, resuhing in the re-establishment of the shallow-water environments throughout northern Istria. Contemporaneous deposits in southern lstria comprise seven facies units, as presented in this paper. They are characterised by a succesion from peritidal deposits (Facies Unit 1), facies with slumps and synsedimentary faults (Facies Unit 2). tempestites (Facies Unit 3), massive carbonate sand bodies (Facies Unit 4), forcsets ofsubaqueous dunes (Facies Unit 5), rudist biostromes (Facies Unit 6) and, finally the re-establishment of the peritidal system (Facies Unit 7). It is very important to determine the inlluence of different factors on the laterally significant change of formerly rather homogeneous, peritidal environments during the Albian/Cenomanian transition, i.e. what was the relationship between sea-level changes, carbonate production and synsedimentary tectonics during formation of the accomodation space. Two, rather weakly expressed episodes of pelagic influence were recorded at all studied localities in the northern and southern parts of Istria, represented by limestones with pelagic microfossils and chert nodules. These episodes are interpreted as a consequence of the shortlasting eustatic sea level rises at the beginning and end of the Middle Cenomanian (VLAHOVIr et al., 1994). Between these episodes, as well as in the younger deposits, there are important lateral differences in the sedimentary record, as described above. Therefore, the question arises as to whether the influence of the global eustatic changes alone, or even

150

supported by different sedimentation rates, was able to produce such differentiation of the formerly more or less flat inner part of the platform. Recent investigations in Istria, especially in its southern part, indicate the important role of synsedimentary tectonics which has significantly modified the effects of eustatic fluctuations on this part of the Adriatic Carbonate Platform. There are already data on the older synsedimentary tectonics of the Istrian part of the Adriatic Carbonate Platform. These include the variable timing of the beginning of the regional Aptian emersion in Istria (starting from the Late Barremian, Early or Late Aptian), clear facies differentiation around the emerged parts, and contemporaneous transgression of deposits of different age during the Middle Albian indicates the important influence of the synsedimentary tectonics (VELIt~et al., 1989), particularly of low-amplitude folding (TIgLJARet al., 1995). MA'nCECet al. (1996) presented new data on a different age for the footwall of transgressive Palaeogene deposits in Istria (from Valanginian to Coniacian-Santonian), and reviewed the influences of Cretaceous synsedimentary tectonics and, therefore the palaeogeographic implications for the Istrian part of the Adriatic Carbonate Platform. For example, Late Cenomanian beds are the youngest Cretaceous deposits in northern Istria, since they are karstified and covered by Palaeogene Foraminiferal limestones (columns 1 and 3, Fig. 6). On the contrary, in southern Istria sedimentation continued to the SantonianCampanian (column 4, Fig. 6), including the Late Cenomanian/Early Turonian eustatic sea-level rise which caused temporary drowning of a large part of the Adriatic Carbonate Platform (JENKYNS, 1991; Guile & JELASKA, 1993; VLAHOVIr et al., 1994). Data presented in this paper also indicates the important influence of synsedimentary tectonics in the production of the accomodation space. By the end of the Albian and the beginning of the Cenomanian in southern Istria a differentiated relief was formed from the previously rather level platform due to synsedimentary tectonics (Fig. 4). Although slope instabilities causing sliding and slumping, which were recorded in Facies Unit 2, may be created by oversteepening or overloading on the ramp through abundant carbonate production, i.e. without tectonic influence, it is important to notice that the immediate footwall of deposits with these features is characterised by typical peritidal deposition. Therefore, these features are the result of the initial stage of the ramp formation (Ist stage, Fig. 4). Inclination of the ramp was perpendicular to the baxis of the macrostructure (the West Istrian anticline striking NNE-SSW), leading to the establishment of laterally different environments, probably in the form of spacious gentle folds. Shallower parts of the ramp formed in this way were characterised by ecological conditions favourable for the extensive growth of rudist colonies, producing a large quantity of material which was subsequently redistributed during high-energy conditions. Lower parts of the carbonate ramp were near the fair-weather wave-.base, in the lower shoreface and offshore-transition zone. This is shown

by the gradual appearance of isolated, thin layers with swaley cross-stratification (SCS), and finally tempestites with well developed hummocky cross-stratification (HCS; PI. 39/3). The progradation of the bioclastic detritus over the depressions resulted in the formation of massive sand bodies and numerous clinostratified bodies representing foresets of subaquaeous dunes, each characterised by the shallowing and fining-upward characteristics. However, the succession in general has a coarsening-upward trend. Rudist colonies were prograding over the completely filled former depressions, i.e. foresets of subaqueous dunes, resulting in the final re-establishment ofa peritidal system over the entire area (3rd stage, Fig. 4). A similar depositional pattern is described in the Albian to Cenomanian inner-shelf basin prograding margin complexes of the Mauddud and MishrifFormations ofAbu Dhabi (ALSHAP,nAN & NAIRN, 1988). BURCHETrE& BRITTON(1985) inferred a very low gradient margin for the Mishrif platform (approximately 2 ~, without slump features), while our study in southern Istria indicates a somewhat higher gradient of the ramp (4-6~ It may be concluded that the most important factors for the facies variability in the Albian and Cenomanian of present Istria were synsedimentary tectonics in the NW part of the Adriatic Carbonate Platform, together with high organic production rates in high-energy environments, resulting in the subsequent destruction, reworking and rapid sedimentation of neighbouring parts. Therefore, accommodation space provided by the gentle synsedimentary tectonics was consumed by the high organic carbonate production resulting in the high sedimentation rate. Interaction of these factors largely masked the influences of global eustatic changes in this part of the Adriatic Carbonate Platform during the latest Albian and Early/Middle Cenomanian. ACKNOWLEDGEMENTS This work was supported by the Ministry of Science and Technology of the Republic of Croatia through Project No. 195-005 and production of the Geological Map of the Republic of Croatia (scale 1:50.000). We wish to thank an anonymous reviewer for critical reading this contribution. REFERENCES ALSHARHAN,A.S. & NAIRN, A.E.M. (1988): A review of the Cretaceous formations in the Arabian Peninsula and Gulf: Part II. Mid-Cretaceous (Wasia Group) Stratigraphy and Paleogeography. - J. Petrol. Geol., 11, 89-112, Tulsa ANASTAS,A.S., DALRYMPLE,R.W., JAMES,N.P. & NELSON,C.S. ( 1997): Cross-stratified calcarenites from New Zealand: subaqueous dunes in a cool-water, Oligo-Miocene seaway. Sedimentology, 44, 869-891, Oxford. BACHOFEN-ECHT,A. (1925): Die Entdeckung von IguanodontenF~hrten im Neokom der Insel Brioni. - Sitzungsanz. Akad. Wiss., Math.-nat. KI., 12, Wien BACHOFEN-ECHT,A. (1926): Iguanodon-F~ihrten auf Brioni. Palaeont. Z., 7/3, 172-173, Berlin BURCHE'rrz,T.P. & BRrrroN,S.R. (1985): Carbonate facies analysis in the exploration for hydrocarbons: a case study from the

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Manuscript received December 15, 1997 Revised manuscript received February 17, 1998